In the manufacture of thin film resistor (TFR) type of thermal ink jet printheads, it is a common practice to photolithographically define the individual heater resistors on a TFR substrate by creating a pattern in an overlying conducting trace layer. This layer is deposited in a predetermined pattern on the resistive heater material using known deposition techniques. The resistive heater layer material may, for example, be tantalum-aluminum, TaAl. The conductive trace pattern is most typically aluminium, although it could also be gold or other conductive material compatible with the other materials in the materials set for the printhead. After the conductive trace material or pattern is completed, it is then usually covered with an inert barrier layer such as a composite layer of silicon nitride and silicon carbide in order to protect the underlying layers from cavitation wear and ink corrosion.
In order to make electrical contact between this conductive trace material and external pulse drive circuitry for the printhead, one standard prior art approach involved etching a relatively large opening or via in the silicon nitride/silicon carbide composite barrier layer and then forming a relatively large contact pad in this opening to thus make contact with the underlying aluminum trace conductor material. Then, wire bonding or pressure contact connections could be made to this relatively large contact pad to provide an electrical current path into the aluminum trace material and to the ink jet heater resistors.
The above prior art structure is possessed with several disadvantages associated with the relatively large opening or via in the insulating barrier layer and directly over the aluminum conductive trace layer. The first of these disadvantages resides in the fact that the large via in the silicon nitride/silicon carbide composite layer exposes a relatively large sidewall area of these materials. This large area sidewall exposure means increasing the area in which pinholes or cracks might possibly occur and thus produce electrical shorts in the barrier layer. As a result of the dissimilarity of the silicon nitride and silicon carbide layers and the differences in their etch rates, there is produced a "diving board" geometry at the edge of these two dissimiliar insulating materials at the via opening. This stepped geometry, when coupled with the large area deposited contact pad in the via, increases the probability of material defects in this region which are capable of reducing wafer processing yields.
Another disadvantage of the above prior art electrical interconnect approach involves exposing a relatively large area of the aluminum trace material in order to provide the desired wide area contact pad thereover. The exposure of such a large area of aluminum trace material in the manufacturing process increases the possibility of forming aluminum oxide, Al.sub.2 O.sub.3, on the conductive trace material and thus rendering it insulating or partially insulating instead of fully conducting.
Another disadvantage of using the above prior art approach resides in the increased probability of undercutting the silicon nitride and silicon carbide layers during the etching of the via therein. Again, such increased probability is caused by the exposure of the relatively wide area sidewall of the silicon nitride/silicon carbide barrier defining the via.
Another disadvantage of using the prior art approach described above relates to the formation of a nonflat dish-shaped contact pad directly over the aluminum trace material. This geometry and structure increases the likelihood of scratching the edge of the printhead structure immediately adjacent the conducting trace material, and such scratching in turn increases the likelihood of producing electrical shorts down through the printhead structure to the aluminum conductive trace material. In addition, the dish shape or non-planar contour of the contact pad makes it difficult to make certain types of electrical connections to the printhead structure, e.g. spring biased pressure connections from a lead frame-type of flexible circuit.
A further disadvantage of using the above prior approach relates to the sensitivity of chipping and cracking at the edges of the multiple layers of materials over which the dish-shaped contact is placed. This chipping and cracking will cause corrosion of these materials at their outer edges, but this does not occur in devices manufactured by the present invention where the lead-in contacts have been removed from pressure contact at the.edges of these interior layered materials.